1. Field of the Invention
The present invention relates to a self-calibrating sensor having a sensor element for electrically detecting a change in physical amount of a workpiece, a signal output means for detecting the change of the sensor element and outputting an electric signal, a processor for conducting calibration calculation of the electric signal for adjusting to the physical amount of the workpiece, and a memory for storing a calibration value to conduct calibration calculation by the processor. The present invention can be applied to, for example, a self-calibration pressure sensor having ASIC (Application Specific Integrated Circuit) including CPU (Central Processing Unit), in which a deformation of a diaphragm in accordance with pressure change of a workpiece can be detected as a change in electro-capacitance.
2.Description of Related Art
Conventionally, a sensor having a sensor element for electrically detecting a change in a physical amount of a workpiece and a signal output means for detecting the change of the sensor chip and outputting an electric signal is used. For example, an electro-capacitance type sensor having a diaphragm deforming in proportion to a change in pressure of the workpiece, a movable electrode formed on the diaphragm, a fixed electrode opposed to the movable electrode and a signal output means for detecting the change of the sensor element as a change in electro-capacitance between the movable electrode and the fixed electrode to output as a capacitance signal is known as such a sensor.
As a specific example, a pressure detector 90 using an electro-capacitance pressure sensor is shown in FIG. 10.
In the figure, the pressure detector 90 has a base member 91 which has a fitting 92 fixedly screwed to a portion to be detected. The fitting 92 is provided with a pressure inlet 93 to which pressure is introduced from an inside of the portion to be detected. The base member 91 has a greatly enlarged diameter remote from the fitting 92 and a pressure sensor 1 is installed thereon to cover the enlarged opening. A sealing member 94 such as an O-ring is inserted between the pressure sensor 1 and the base member 91 in order to ensure sealability therebetween.
The pressure sensor 1 has a diaphragm 1A on a surface facing the pressure inlet 93, the diaphragm 1A receiving the pressure from the pressure inlet 93 to displace in a direction intersecting the surface. The pressure sensor 1 outputs the displacement of the diaphragm 1A by an internal electrode (not shown) as a change in electro-capacitance.
A processor 5 is mounted on the pressure sensor 1 on a side opposite to the fitting 92. The processor 5 is connected to an electrode of the pressure sensor 1 through a through-hole etc. provided on the pressure sensor 1 to receive a signal showing the change in the electro-capacitance and to output to the outside after amplifying and conducting predetermined arithmetic processing etc.
An output substrate 95 is provided for outputting the signal from the processor 5 to the outside. The processor 5 and the output substrate 95 are wired by wire-bonding etc. and a durable cable 96 is wired to connect the output substrate 95 and the outside.
The base member 91 is covered by a cover member 97, in which all of the pressure sensor 1, the processor 5, the output substrate 95 etc. are accommodated.
The change in the pressure of the workpiece and the capacitance signal do not always linearly correspond within the entire measurement range. Furthermore, slight deviation is caused to respective products when the sensors are mass-produced as consumer products.
Accordingly, calibration work is conventionally conducted for every electro-capacitance pressure sensor so that highly accurate measurement is possible irrespective of the electro-capacitance sensor employed and the measurement range.
However, an outside adjusting apparatus such as potentiometer and trimming apparatus as well as the sensor is necessary for such calibration work and the calibration work is necessary to be done to individual electro-capacitance pressure sensor, thereby largely increasing the cost required for the calibration work after manufacturing the sensor.
In view of the above, a self-calibration pressure sensor is proposed, in which an electro-capacitance pressure sensor has ASIC including CPU which performs the above calibration work.
FIG. 11 shows an example of the conventional self-calibrating sensor.
Self-calibrating sensor 100 has a sensor element 101 including a diaphragm, a signal output means 102 for outputting a change in electro-capacitance in accordance with the pressure change of the sensor element 101 as a capacitance signal S1, a processor 103 for conducting calibration calculation of the capacitance signal S1 to adjust to the pressure of the workpiece, and a memory 104 for storing calibration value for conducting calibration calculation by the processor 103.
When the pressure of the workpiece is measured by the self-calibrating sensor 100, the processor 103 obtains the calibration value for conducting calibration calculation from the memory 104 simultaneously with activation of a power supply unit 105 connected to the self-calibrating sensor 100. Subsequently, the processor 103 conducts calibration calculation of the capacitance signal S1 outputted from the signal output means 102 based on a predetermined calculation formula for outputting signal of pressure converted value P.
When the calibration value of the self-calibrating sensor 100 is calculated, a known calibration pressure P0 is applied to the sensor element 101 and the capacitance signal S1 outputted by the signal output means is detected. The calibration value is set so that the pressure converted value P equals P0 in the calculation formula for obtaining pressure converted value P based on the outputted capacitance signal S1.
For calibrating the self-calibrating sensor 100, a pressure generator 110 is connected to the sensor element 101 of the self-calibrating sensor 100 and the processor 103 of the self-calibrating sensor 100 and a controller (not shown) of the pressure generator 110 are electrically connected to a computer 120, as shown in FIG. 11.
A control signal S2 for outputting calibration pressure P0 is outputted from the computer 120 to the pressure generator 110 and the capacitance signal S1 outputted from the self-calibrating sensor 100 is received by the computer 120 through the serial interface 106. The computer 120 sets the calibration value of the self-calibrating sensor 100 by processing the calibration pressure value P0 based on the outputted control signal S2 and the received capacitance signal S1. The determined calibration value is outputted to the self-calibrating sensor 100 from the computer 120 and is stored in the memory 104 through the serial interface 106.
However, following disadvantage occurs in the conventional self-calibrating sensor.
For outputting the calibration value set by the computer 120 to the self-calibrating sensor 100, an exclusive I/O (input/output) line has to be provided. Specifically, as mentioned above, the serial interface 106 and the like has to be connected to the self-calibrating sensor 100 by a serial and parallel cable for outputting the calibration value, or digital signal has to be coupled to a power supply line to transmit the calibration value.
In the actual pressure measurement, the computer 120 checks whether the calibration value is stored in the memory 104 through the exclusive I/O line to determine which one of the actual measurement and calibration work should be done according to presence of the calibration value. Accordingly, though the aforementioned exclusive I/O line is only used for the calibration work or the check before conducting measurement, the serial interface 106 has to be separately provided and the computer 120 and the self-calibrating sensor 100 has to be connected via a cable.
Therefore, an extra portion is caused to measurement system including the self-calibrating sensor 100 and the computer 120, thereby increasing the cost for structuring the measurement system. Further, since the exclusive serial interface 106 has to be provided to the self-calibrating sensor 100, the production cost for the self-calibrating sensor 100 is also increased.
An object of the present invention is to provide a self-calibrating sensor having a sensor element for electrically detecting a change in physical amount of a workpiece, a signal output means for detecting the change of the sensor element and outputting an electric signal, a processor for conducting calibration calculation of the electric signal to adjust to the physical amount of the workpiece, and a memory for storing a calibration value for conducting calibration calculation by the processor, in which the exclusive I/O line can be omitted to reduce the production cost and further reduce the cost of the measurement system.
For attaining the aforesaid object, the self-calibrating sensor according to present invention additionally has (1) a calibration value calculating circuit for calculating a calibration value and (2) a calibration completion determining circuit for determining whether the calibration value calculation is completed or not.
Specifically, a self-calibrating sensor according to the present invention has a sensor element for electrically detecting a change in physical amount of a workpiece, a signal output means for detecting the change of the sensor element and outputting an electric signal, a processor for conducting calibration calculation of the electric signal to adjust to the physical amount of the workpiece, and a memory for storing a calibration value for conducting calibration calculation by the processor. The self-calibrating sensor is characterized in that the processor includes a calibration value calculating circuit for calculating calibration value for conducting the calibration calculation from the electric signal outputted by the signal output means when a known calibrating physical amount is applied to the sensor element, a calibration arithmetic circuit for obtaining converted value of the physical amount by conducting calibration calculation based on the electric signal detected against unknown measured physical amount by using the calibration value calculated by the calibration value calculating circuit, and a calibration completion determining circuit for determining whether the calculation of the calibration value by the calibration value calculating circuit is completed, that a flag signal indicating whether the calibration value calculation by the calibration value calculating circuit is completed is stored in the memory together with the calibration value, and that the calibration completion determining circuit detects the flag signal for determining whether the calculation of the calibration value is completed or not.
According to the present invention, since the self-calibrating sensor has the aforesaid calibration value calculating circuit, the calibration value can be calculated inside the self-calibrating sensor, thereby eliminating the need for the connection of the self-calibrating sensor to the outside computer etc. further since the self-calibrating sensor has the aforesaid calibration completion determining circuit, the completion of the calibration value calculation can be judged inside the self-calibrating sensor. Accordingly, it is not required that the self-calibrating sensor is connected to the outside computer to check the completion of the calibration work by the outside computer.
Therefore, the self-calibrating sensor and the outside computer are not required to be connected by the exclusive I/O line for the calibration work, thereby reducing the cost for the measurement system including the self-calibrating sensor.
The serial interface etc. for connecting the I/O line for the calibration work are not required for the self-calibrating sensor, thereby reducing the production cost of the self-calibrating sensor. Further, since the calibration completion determining circuit judges the completion of calibration value calculation by the flag signal stored in the memory, whether the calibration value is calculated or not can be immediately detected in initiating the measurement by the self-calibrating sensor, thereby speeding up the judgement by the calibration completion determining circuit.
In the above arrangement, the calibrating physical amount is normally applied to the sensor element in a plurality of times and the calibration value calculating circuit calculates the calibration value based on calibrating physical amount applied in the plurality of times.
The conversion from the electric signal S such as a capacitance signal to pressure converted value P is conducted by a transforming formula. For instance, when the transforming formula is set as the simplest P=aS+b, the calibration value calculation is to calculate coefficients a and b of the above formula. The pressure converted value can be calculated based on the electric signal by obtaining the coefficients a and b. The coefficients a and b can be calculated by binary simultaneous equations set by two calibrating physical amounts P0 and P1 and electric signals S0 and S1 detected at that time period. Incidentally, the number of the coefficients increases as the transforming formula gets complicated, so that the calibrating physical amount is applied to the sensor element in an increasing number of times.
In order to apply the calibrating physical amount to the sensor element in a plurality of times, the calibrating physical amount may be applied in the plurality of times synchronizing with the on/off operation of the power supply unit of the self-calibrating sensor, or alternatively, the calibrating physical amount may be applied in the plurality of times at a predetermined interval from power-on operation of the power supply unit. Both arrangements are possible for the present invention.
When the calibrating physical amount is applied in the plurality of times synchronizing with the on/off operation of the power supply unit, the aforesaid processor preferably includes power-on counter for storing the power-on count of the calibrating physical amount in the memory.
Since the power-on count is stored in the memory by the power-on counter, the progress of the calibration work can be checked by referring to the power-on time stored in the memory by the calibration value calculating circuit, thereby automating the calibration work.
As the memory, a non-volatile memory of which stored information is not lost when the power supply unit is shut off is preferably used. For instance, EPROM (Erasable and Programmable Read Only Memory) and E2PROM (EEPROM, Electrically Erasable and Programmable Read Only Memory) may preferably be used.
By adopting the non-volatile memory as the memory, the stored information is not lost by the on/off operation of the power supply unit. Accordingly, information necessary after the calibration work such as the calibration value and flag signal indicating whether the calibration work is completed or not can be permanently stored for every self-calibrating sensor. Further, repeated calibration work etc. can be made unnecessary and, during the calibration work, intermediate information such as the power-on count can be retained irrespective of the on/off operation of the power supply unit.
When the calibrating physical amount is applied in the plurality of time at a predetermined interval from the power-on operation of the power supply circuit, the calibration value calculating circuit preferably calculates the calibration value from the calibrating physical amount applied in the plurality of times.
Since the calibration pressure is sequentially applied within a predetermined time period after the power-on operation of the power supply unit unlike applying the calibration pressure in synchronization with the on/off operation of the self-calibrating sensor, the intermediate information for calculating the calibration value is not required to be stored in the non-volatile memory and the like, thereby minimizing the information stored in the non-volatile memory. Further, since the plural on/off operations of the power supply unit are not required, the load applied to the other circuit of the self-calibrating sensor in accordance with the on/off operation can be minimized in conducting calibration work.